1. Field of the Invention
The present invention relates to an injection timing control device for a fuel injection pump, and more specifically, to an improvement in structure of an injection timing control device for a fuel injection pump where a fuel injection timing is variable.
2. Description of the Prior Art
In distributor type fuel injection pumps for use in the diesel engines or the like, a plunger is rotated by means of a drive shaft rotating synchronously with the engine, so as to achieve switching among engine cylinders for a subsequent fuel injection. Further, following the rotation of the drive shaft, convex portions of a face cam at the base end of the plunger ride on rollers of a roller ring to advance the plunger so that fuel is fed under pressure to a corresponding fuel injection valve from a fuel pressure chamber and thus the fuel injection into the corresponding engine cylinder is started. Changing of the fuel injection timing is achieved by moving a timer piston coupled to the roller ring, depending on a pressure of fuel from a fuel feed pump, so as to rotate the roller ring to change positions of the rollers. Japanese First (unexamined) Utility Model Publication No. 63-110640 shows such a fuel injection timing control device for a fuel injection pump. Japanese First (unexamined) Patent Publication No. 5-332170 further proposes a fuel injection timing control device for a fuel injection pump, which aims to prevent the partial abrasion of a servo valve which moves depending on the fuel pressure from the fuel feed pump to control a position of the timer piston.
On the other had, reflecting the recent highly advanced engine control technique, those fuel injection timing control devices have been available, wherein the fuel injection timing can be adjusted desirably according to a required engine control. FIG. 10 shows one example of such fuel injection timing control devices. In FIG. 10, a timer cylinder 94 is provided in a housing 92 under a roller ring 91. The timer cylinder 94 is dosed at its opposite axial ends and communicates with a connecting opening 93 which further communicates with a fuel chamber. A timer piston 95 is slidably received in the timer cylinder 94. The roller ring 91 is connected to the timer piston 95 via a slide pin 991 extending from the roller ring 91 and a spherical bearing 992 which is rotatable in the timer piston 95. When the timer piston 95 moves rightward and leftward in the figure, the roller ring 91 rotates in normal and reverse directions. A timer high-pressure chamber 94b communicates with the connecting opening 93 via a passage 951 formed in the timer piston 95 so as to receive the fuel at about 5 atm. from the fuel chamber via a flow restrictor 952 provided in the passage 951. On the other had, a timer low-pressure chamber 94a communicates with an inlet port of a fuel feed pump (not shown) via a passage 98 so as to be constantly held at the atmospheric pressure. In the timer low-pressure chamber 94a is disposed a spring 953 for biasing the timer piston 95 toward the timer high-pressure chamber 94b.
Further, a passage 96 is provided under the timer cylinder 94 for establishing communication between the timer high-pressure chamber 94b and the timer low-pressure chamber 94a. An oil-pressure control valve 97 is further provided to control a flow rate of the fuel flowing in the passage 96 so as to adjust a fuel pressure in the timer high-pressure chamber 94b. The timer piston 95 is controlled to a position where the fuel pressure in the timer high-pressure chamber 94b and the sum of the fuel pressure in the timer low-pressure chamber 94a and the biasing force of the spring 953 are balanced. The control of the oil-pressure control valve 97 is achieved by changing a duty cycle of a pulse signal at, for example, 40 Hz, that is, by changing a rate of time period of energization to the oil-pressure control valve 97.
Following the strengthening of exhaust gas regulation, the demand has been increased for a wider range of fuel injection timing variation, stabilization of the fuel injection timing, and improvement in response characteristic for changing the fuel injection timing.